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Water, Water Everywhere

The majority of the fresh water we consume comes from either surface or groundwater sources. Surface water is, of course, an obvious choice. Rivers and lakes are all around us and are constantly replenished by precipitation and glacial melt. In many cases, surface water is not an option, either because it doesn’t exist in large enough volumes or due to quality concerns. Then we drill wells and use water from the ground – water stored in the pores or empty spaces within rocks and sediments. To give you an idea of volume, lakes and rivers combined contain roughly 93,000 cubic km (22,500 cubic mi) of water globally, while fresh groundwater aquifers (reservoirs) account for 10.5 million cubic km (2.5 million cubic mi). That said, fresh water reserves are not evenly distributed across the globe, and particularly in the case of groundwater, are not all accessible. This is why countries such as Israel and Saudi Arabia are forced to obtain part of their water through desalination of seawater – an energy-intensive and costly process.

Swamps often form where the water table (groundwater level) meets the surface

However, a recent paper in Nature has found fresh water in the most unlikely of places: beneath the ocean floor. The idea in itself is not a new one. As far back as the 1970s, drilling off the coast of New Jersey found that water stored in the rocks of the continental shelf (the submerged edge of the North American continent) had only a quarter of the salt content of seawater. The measure we use to show salinity is the TDS or total dissolved solids. The TDS of fresh water is less than 1 gram per liter, while seawater falls between 30 and 50 grams. Water in the range of 1 to 10 is called brackish water, something like what you’d find in a swamp.

The study has identified what it calls VMGRs orvast meteoric groundwater reserves (‘meteoric’ being just a fancy word for ‘from precipitation’). These VMGRs are found beneath continental shelves across the world and have a TDS of less than 10 grams per liter. In fact, they estimate that globally, VMGRs store 300,000 cubic km (70,500 cubic mi) of fresh water alone. To put this in perspective, that is a hundred times more water than all groundwater extracted since 1900. While not infinite, this does potentially open vast new reservoirs for coastal cities to use. Even brackish water is of value, since it is much cheaper to desalinate than seawater.

This map, taken from the Post et al. paper, shows the locations of several large VMGRs

Let’s finish off with some geology. During the last glacial maximum between 26 and 19 thousand years ago, sea level was much lower than it is today. Thus the continental shelves were exposed and actively recharged by lakes and rivers. Today’s VMGRs are remnants of groundwater aquifers that formed during this time, which became submerged once sea levels rose after the last ice age. What this does mean is that, for the most part, these reservoirs are not renewable. For the moment, scientists will continue to collect data to confirm the exact size and locations of these reservoirs and their interactions with seawater. But in the not-so-distant future, you might just find that the water flowing through your tap has traveled all the way from below the ocean floor.

If you aren’t able to view the full Nature article, you can read a summary here.

Be warned: you may have just sustained a lethal dose of mostly harmlessscience.

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